No one can tell you the exact right settings because much will depend on the light in the church. The time of day, the type of windows and their placement, if it's overcast or sunny.. they will all affect the shot and the settings. And we all have our own approach to how we shoot so there is no right tor wrong anyway, only a properly exposed image and there are different ways to get that.
Keeping the lens wide is great advice. The longer the lens, the more light it needs to get the exposure. When it's open wide to 18mm you should be able to get a decent shutter speed with no flash on the XT in many in door situations if the room is well lit.
If it were me.... I would go with shutter priority (Tv) in this type of situation. I'd set it to 1/125 and have the ISO at 1600. Yes, it will be noisy on the XT, but with the lens you have that's probably unavoidable. I'd keep the lens pulled in and let the camera choose the aperture, it's likely going to be pretty wide. If that can't get the exposure with that shutter speed I would drop down, but not below 90 in a situation like that. If there was more than enough light to get the shot at 1/125, I'd either zoom in some or lower the ISO, depending on the situation. And keep in mind, the shutter speeds I'd start with are with the lens at 18mm so if you zoom in much you will have to increase the shutter speed ot compensate for the longer focal length.
It's really a balancing act.
Also.. the XT does not auto focus well in low light. If it's particularly muddy light I might use manual focus. Again though, not something I can say for sure sitting here at my computer and not on site.